1. Field of the Invention
The invention relates to a method of outside of mold sintering of a shaped inorganic article or preform as to produce a transparent aluminum oxynitride product. More particularly, the invention relates to a method of outside of mold sintering a first heat treating step and a second sintering step, performed at a reduced temperature. The invention also relates to an aluminum oxynitride product formed by the method of the invention.
2. Discussion of the Prior Art
Transparent ceramics and especially ceramics that transmit light in the visible and infrared range are used in both commercial and military applications. Examples of these products include orthodontic brackets, gas tight lamp envelopes, radar domes, windows for infrared sensors, chemical processing equipment, refractory and ballistic armor.
The use of polycrystalline aluminum oxynitride for these products has been limited by the frequency of flaws found in the products made by known methods and the current cost of manufacturing.
The attainment of substantial transparency in a high density, polycrystalline aluminum oxynitride body is dependent upon a number of factors. These factors are related by the formula:I/Io=e−ad  (1)where:
Io is the intensity of incident visible light,
I is the intensity of transmitted visible light,
a is the absorption coefficient of the material, and
d is the thickness of the specimen.
The absorption coefficient of the specimen is used to account for the natural transmissivity of a perfect specimen of the material and also all light scattering impurities and imperfections in a sample. Impurities may include any amount of a second phase, such as one or more liquid, glass or noncrystalline phases, in or around a solid grain. Imperfections may include voids trapped at grain boundaries and within grains. Imperfections include grain boundary cracks resulting from abnormal grain growth or from excessively large rates of grain growth.
Polycrystalline, cubic aluminum oxynitride is used in commercial and military applications because it is a transparent ceramic and has other desirable physical properties. U.S. Pat. No. 4,241,000 to J. W. McCauley et al., herein incorporated by reference in its entirety, teaches a conventional process for making this material. In the process, precursor powders are first mixed and then simultaneously reacted and sintered to produce the resulting single phase, polycrystalline, cubic aluminum oxynitride body. These bodies are reported to have a density of 99% of theoretical density and an in-line transmission of at least 50% in the wavelength range of 0.3 to 5 microns.
The manufacture of aluminum oxynitride bodies for the industrial and military purposes cited above has been hindered by the frequency of flaws found in the products made by conventional methods. The physical and chemical flaws have shown effect on the optical transparencies of the resulting bodies.
The flaws that affect optical properties are attributed to a number of causes. Such flaws generally result in differing optical properties throughout an otherwise uniform product. A first major cause of optical flaws is the lack of homogeneity of inorganic substituents (chemical processing aids) throughout the product. Non-homogeneity may be caused by insufficient mixing of constituents prior to sintering.
A second significant cause of optical imperfections has been found to be the presence of impurities such as glasses or voids in or around the crystalline grains. Voids, sometimes referred to as pores, have irregular boundaries that may scatter light. Additionally, the refractive index of any gas contained in a pore or any glass in solid solution in or around a grain is significantly different from that of the grain itself. For example, air has a refractive index of 1.0, whereas the refractive index of flawless aluminum oxynitride ranges around 1.785. The measured value of refractive index depends on the relative amount of aluminum nitride indicated by the value of x in the chemical formula for aluminum oxynitride (See Formula 2. below).
Conventionally, aluminum oxynitride products are produced commercially in a process described in U.S. Pat. No. 4,520,116 to Gentilman et al.; U.S. Pat. No. 4,686,070 to Maguire et al.; and U.S. Pat. No. 4,481,300 to Harnett et al., each of which is hereby incorporated by reference in its entirety. Therein is described a process by which (1) carbon and aluminum oxide are mixed for several hours; (2) the mixed powders are heated to a temperature of 1200° C. for several hours to reduce the surface of the aluminum oxide to aluminum nitride; (3) the powders are reacted at a temperature between 1550° C. and 1850° C. in flowing nitrogen for several hours to fully react the powders to form aluminum oxynitride; (4) the reacted powder is ball milled in methanol for 16 hours to break down hard agglomerates; (5) the dried powder is heated in air to 600° C. for 2 hours to burn off residual organic material; (6) sintering aids, e.g. B, La2O3, are added to the AlON powder and mixed; (7) a green body is formed using conventional ceramic forming techniques; and finally (7) the green body is sintered at a temperature between 1900° C. and 2140° C. for dwell times between 24 to 48 hours.
Therefore, there exists the need in the art for a simplier process capable of producing an aluminum oxynitride product which is transparent.